In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high device densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels) on semiconductor wafers. In order to accomplish such densities, smaller feature sizes and more precise feature shapes are required. This may include width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry, such as corners and edges, of various features. The dimensions of and between such small features can be referred to as critical dimensions (CDs). Reducing CDs and reproducing more accurate CDs facilitates achieving higher device densities.
High resolution lithographic processes are used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist. The film is selectively exposed with radiation (e.g., optical light, x-ray, electron beam, etc.) through an intervening master template (e.g., mask, reticle, etc.) forming a particular pattern (e.g., patterned resist). Dependent upon coating type, exposed areas of the coating become either more or less soluble than unexposed areas in a particular solvent developer. More soluble areas are removed with the developer in a developing step, while less soluble areas remain on the silicon wafer to form a patterned coating. The pattern corresponds to either the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
The achievement of smaller critical dimensions is related to the resolution of the lithographic system. In particular, resolution can be defined as:resolution=kλ/NA where k is a lithographic constant, λ is an exposure radiation wavelength, and NA is a numerical aperture. Numerical aperture is defined as a lens's ability to gather diffracted light and resolve fine details onto a substrate. Numerical aperture can be derived as follows:NA=n sin αwhere n is a refractive index and 2α is an angle of acceptance of a lens. Refractive index is defined as a ratio of the speed of light in a vacuum to the speed of light in a particular medium.
From the above relationships, it can be seen that resolution can be increased by increasing refractive index and/or decreasing lithographic constant. Efforts to increase resolution and thereby reduce critical dimensions can be accomplished by several approaches. One approach involves the reduction in wavelength of the exposure radiation such as is achieved by moving from mercury g-line (436 nm), to excimer laser (193 nm), and further to 157 nm and extreme-ultraviolet (EUV) wavelengths. A second approach involves the utilization of resolution enhancement techniques such as phase-shifting masks. The use of phase shifting masks and off-axis illumination techniques have led to a reduction in the lithographic constant from about 0.6 to about 0.4. Finally, a third approach increases the numerical aperture through improvements in optical designs, manufacturing techniques, and metrology. Such improvements have lead to increases in numerical aperture from approximately 0.35 to greater than 0.7.
Immersion lithography provides another approach for increasing the resolution of an optical lithographic system and thereby achieving smaller critical dimensions. In immersion lithography, the gap between a substrate (e.g., wafer or reticle) and a final optical component (e.g., lens) is filled with an immersion medium which has a higher refractive index than the refractive index of air. Utilizing an immersion medium with a refractive index greater than that of air (approximately 1) increases numerical aperture. Increasing numerical aperture increases the resolution of an optical lithography system and thereby facilitates achievement of smaller critical dimensions. Furthermore, utilization of an immersion medium can decrease an effective wavelength of an exposure radiation propagating within the immersion medium without changing exposure sources, lasers, lens materials, etc.
Currently, immersion lithography is limited by various characteristics of immersion mediums and the immersion lithographic process. One significant problem encountered in immersion lithography is that most photoresists release gas upon exposure. Such released gases can form bubbles in the immersion medium. These bubbles can interfere with the exposure radiation resulting in scattered exposure light and potential processing defects in a substrate. Systems and methods which can detect bubbles in the immersion medium and accordingly alter processing can significantly improve the efficacy of immersion lithography systems and processes.